A backlight including a plate-like light guide plate is disposed on a back surface of a liquid crystal panel of a liquid crystal display device in some cases. In the light guide plate, one main surface having a large area is a light emission surface, the other main surface facing the light emission surface is a reflective surface, and at least one lateral surface orthogonal to the main surface of the light guide plate is an incident surface. A light source such as an LED is disposed to face the incident surface. Light emitted from the light source enters the light guide plate from the incident surface and propagates through the light guide plate while repeating reflection. When a part of the light strikes dots or the like formed on the reflective surface, a propagation direction of the part of the light is changed, and the light is partially emitted from the light emission surface. The remaining light propagates toward the lateral surfaces of the light guide plate. Therefore, to improve a luminous efficiency on the light emission surface, it is known that a reflective member is disposed to face the lateral surface of the light guide plate, so that the light supposed to be emitted from the lateral surfaces is returned to the light guide plate.
However, when the reflective member is disposed to face the lateral surface of the light guide plate, the light propagating toward the lateral surface and the light reflected and returned by the lateral surface, are concentrated near the lateral surface, and thus the following phenomenon occurs: the luminance near the lateral surface of the light emission surface is more increased than the other regions.
In order to overcome such non-uniformity of luminance, Japanese Unexamined Patent Publication (Kokai) No. H6-289231 discloses that a gap is formed between the lateral surface of the light guide plate and the reflective member, that a bottom surface near the lateral surface is formed as a mirror surface, and that the reflective member is formed as a concave mirror.